Been listing a collection of D&D and AD&D modules. There were a couple of green module A1s. One of them was a 1st so I got a chance to compare the printings. 1st prints aren’t super rare, but neither are they common.
Slave Pits of the Undercity
1980 … David Cook … 24 pages … TSR 9039 … ISBN 0935696253
Sources for A1 “Slave Pits of the Undercity”:
The Acaeum is my go-to for early Dungeons & Dragons printing data. The site entry for module A1 notes of a first edition: “Some of the interior illustrations have small circular blemishes on them (on pages 10, 12, and 14).”
Later printings removed the artifacts.
Above is page 12 from both printings. 1st print top, 3rd print below.
I quickly discovered that the blemishes are faint, and don’t show up well in photos.
So I took some close-ups. Below is the full illustration from page 10 of the 1st print, and the close-up.
I’d say the blemishes are barely noticeable, except on Page 12. But a detail they felt worth fixing, so there’s that.